text:
What is the most common reason to deploy a parachute?
On aircraft equipped with parachute, What is the most common reason to deploy a parachute?
Is it engine failure, loss of control, disorientation, out of fuel, etc. or something else?
Clarification: The question is limited to only the deployments in the air  for emergency reasons. I.e.: Parachutes for braking purposes in the ground is excluded.

Per the answer posted, you may want to clarify that you are asking about light civil aircraft that use parachutes for emergencies.  At least that's what I assume you are asking about, but since some older military jets used a parachute on every landing to slow down someone may include this in an answer.

Per this Flying Magazine.com article, dated 2015 (yes, I know, old), BRS claimed 300+ pilots and passengers survived parachute decent, and COPA (Cirrus Owners and Pilots Assc) claimed 95 survived 46 chute pulls. Both claimed no deaths related to the deployments, but the article has no details, so not an answer.

I went through the list of CAPS deployments on the Cirrus Owners & Pilots Association site linked by John K. I ignored the two that deployed on parked aircraft due to thunderstorms which leaves 148 intentional deployments. Not all deployments were successful (most were), not all occupants survived (most did), but this catalogs the reason they chose to deploy the parachute. To date, by far the most common reason for parachute deployments is loss of engine power for some reason.
70 deployments (48%) due to an engine problem of some kind, and a further 16 (11%) due to some sort of fuel issue, so a total of 86 (61%) deployments were due to a loss of engine power for some reason. While a few engine issues were due to pilot error, most were mechanical issues. Fuel issues were a combination of exhaustion due to pilot error (running out of fuel, not switching from an empty tank), issues when switching tanks, and mechanical issues.
The next largest issue is due to loss of control, resulting in 30 deployments (20%). Besides failures to maintain adequate airspeed, this includes disorientation and misuse of the autopilot, either in VMC or IMC (visual or instrument meteorological conditions).
Weather directly accounts for 13 of the deployments (9%). 3 (2%) were simply loss of control due to severe weather, 7 (5%) were due to icing, and another 3 (2%) due to VFR in IMC (visual flight into bad visibility).
Structural failure accounted for 7 of the deployments (5%). 5 (4%) of these were due to midair collisions while 2 (1%) were due to control surfaces departing the aircraft.
Instrument failures were the reason for 3 deployments (2%).
Pilot incapacitation was the reason for 2 deployments (1%).
7 of the deployments (5%) don't seem to have sufficient information to determine a reason at this time.

Here is a stacked area chart of the different reasons over time. The early data is a bit crazy as the deployments don't really pick up much until around 2010.

Here is a comparison of the total deployments and the total Cirrus aircraft delivered.

Lastly, here is a chart of the individual reasons charted over time. I mostly include this to point out that engine problems become much more common around 2013 while other reasons maintain more consistent rates. It's also been a while since a fuel exhaustion or icing incident.

Very nicely done @fooot! Thank you for putting in that effort.

Cirrus's system is probably the most common parachute recovery system outside of ultralights.
You can review 149 Cirrus deployments here and catalog the reasons and have some hard data. I wasn't able to find a detailed breakdown, but the information is all there if you want to do the analysis yourself.
Perusing the site, I see a few IMC (Instrument Meteorological Conditions, otherwise known as blundering into cloud when you have no business being there) encounters and lots of loss of power problems, either engine failures or running out of gas, and a surprising number of odd events like, say, a pilot who apparently ran out of gas because he was stoned.
With a fast landing plane like the Cirrus, a lot of people are going to choose the parachute over a forced landing compared to an ultralight, where most of the deployments are going to be due to control problems or structural failure.

Definition of IMC please?

Sorry. Instrument Meteorological Conditions.  In other words, getting caught in clouds where you lose sight of the ground or horizon, when not trained for it.

On aircraft equipped with parachute, What is the most common reason to deploy a parachute?
Is it engine failure, lost of control, disorientation, out of fuel, etc. or something else?

None of the above. The most common reason to deploy a parachute on aircraft equipped with a parachute is that deploying the parachute is part of a landing procedure.
The currently accepted answer states that the most common reason is engine or fuel problems with 86 incidents. However, the Space Shuttle orbiters alone deployed their brake parachutes 86 times during landing.
If you add to that the B-52, Sud Aviation Caravelle, Tupolev Tu-104, Tu-124, Tu-144, Learjet 25 and 35, Dassault Falcon 20 and Mirage, several glider models, BAe Hawk, Norwegian F-15s, Eurofighter Typhoon, F-100, F-104, F-105, F-106, F-117, F-16, F-4, F-5, F-86, SR-71, Handley Page Victor, MiG-21, MiG-29, MiG-31, Su-22, Su-24M, Su-25, Su-27, Su-30, Su-34, Hawker Hunter, and various X-planes such as the XB-70, XB-51, X-10, and prototypes such as the early Concorde, all of which use braking parachutes at least some of the time, you will probably find that the number of times airplanes deployed parachutes to slow down during or after landing absolutely dwarfs the number of times airplanes deployed parachutes because of engine problems.

I've added the clarification to the question that is is only for in the air deployment and for emergency reasons only.

@Gabe - it's good that you added the clarification, although to me it seemed obvious from your original question that this is what you meant since you listed "engine failure, lost of control, disorientation, out of fuel, etc." as possible causes, which would not be the case when using braking parachutes on normal landings. There might be situations where that type of aircraft is landing on a runway in conditions that would not normally need to use the chute, but in an emergency landing for say mechanical problems maybe they would. But still that would not be the most common reason.

@StevePemberton: That is interesting, I did not know that about Aviation.SE. On most of the other sites I frequent, it would be the opposite: editing a question in a way that invalidates existing answers would not be considered "good", but in fact, not appropriate. I found this Meta discussion (https://aviation.meta.stackexchange.com/a/3104/3721), but it was inconclusive, although the highest-voted and accepted answer seems to indicate that is the case on Aviation.SE as well.

Jörg  - I agree that in many cases it's unfortunate that a vaguely worded question can result in answers that don't fit as well when the question is later clarified. However new contributors tend to use wording that takes some parsing. In this case the question title could have meant anything including sport skydiving parachutes. Then the first sentence in the description narrowed it down to aircraft parachutes. The next sentence (in my opinion) narrowed it down to whole-plane recovery systems. More experienced contributors probably would have included whole-plane recovery system in the title.

That's a nice list that you posted, several aircraft on there that I didn't realize had parachutes. As a side note I got to see the first Shuttle parachute deployment STS-49 at Edwards in 1992. Although I was on the other side of the lake bed in the non-VIP viewing area, so all I saw was a red dot pop up behind the Shuttle. But still it was pretty cool.


text:
How many pounds of thrust can a TBM 900 pull at full thrust?
Curious of how many lbs/thrust can a single engine like the Socata TBM 900 (or a similar airplane) pull at sea level while parked at full thrust?

The Daher Socata TBM has a climb rate of 2200 fpm at 124 knots and a glide ratio 1 to 10.  This all works out to a thrust to weight ratio of 0.275

What propeller? Without one thrust is limited to what is produced by the exhaust of the engine.

In the Pilots Operating Handbook there are 2 clues to thrust output:  glide ratio and climb performance.
Glide ratio gives thrust requirement for level flight at Vbg.
For a Cessna 172 this is about 1 to 9.  For a weight of 2400 pounds this is around 270 pounds of thrust.
Climb performance at Vy, which is fairly close to Vbg, is around 730 feet per minute.  At a forward speed of 70 knots we have 730/(70×6076/60)×2400 = 247 lbs climbing thrust.
Add the two together and divide by 160 horsepower equals 3.23 pounds per horsepower.
That would be for a fixed pitch, and thrust output is a function of engine power × prop efficiency factor, which will vary with airspeed.
The 900 TBM has a glide ratio of 1 to 10 at 7000 pounds:
700 pounds thrust for level flight.
It climbs at 2200 feet per minute with a climb thrust of: 2200/(124×6076/60)×7000 = 1226 pounds
(700+1226)/850 = 2.26 pounds per horsepower.
The prop is 91 inches vs 75 inches for the 172.  It has 5 blades spinning at 2000 rpm vs 2 blades for the 172 (at a slightly higher rpm).
Though the lower thrust per horsepower efficiency may be surprising (should it be?), the fuel economy cross-over to jets is less.

You can ballpark it. A decently efficient constant speed propeller makes around 5lbs per horsepower, give or take. The PT6-66 in the 900 is 850 HP, so you could say somewhere around 4000-5000 lbs is a  reasonable guesstimate.

The thrust estimate may be a bit high.  The plane itself is only about 7400 lb at max TOW, and the take-off acceleration initiated at nearly 0.7g from thrust of this propeller would be remarkable.

I was going to say the same but since I'm not an expert in the area can't speak too much ....  I find it high even though it is indeed a high performance airplane....

A more conservative number would be about 4.5 lb/hp. 3800 lbs.

@Gabe there is a video online where a guy with a Cessna 140 takes an actual thrust measurement. 85hp Cont, fixed pitch prop. He measured 300lbs, or 3.6 lbs/hp. Prob is with a fixed pitch the engine isn't making 85 hp static, maybe 75, until it gets moving and it can wind up to redline RPM, and the prop's efficiency is poor when static. A CS prop will be at redline while static and is running at much higher efficiency with its blades optimized for the torque. 4.5 to 5.5 lbs/hp is not unreasonable.

I would definitely agree power would be greater at higher altitude on a thrust to TAS basis.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is the quietest propeller aircraft in the GA category?
Among all propeller aircraft in General Aviation which one has the lowest noise level (from outside the aircraft)?
Wondering what makes it quiet(is it just the prop, are there other factors, etc)?

Are you willing to consider Human-Powered Aircraft?  With no engine or exhaust noise and a big, (comparatively) slow-moving propeller, these are probably unbeatable.  If you want not just a propeller but also an internal combustion engine, then that's a different consideration.

@RalphJ I didn't see that one coming, indeed that could be the quietest of all time. I want to limit it to the GA category....updating post.

How  strict are you on the concept of GA?
https://en.wikipedia.org/wiki/Lockheed_YO-3_Quiet_Star

@RobMcDonald that one definitely qualifies even though it has a military mission I think it well suited to be on the GA world, very interesting one...

Does the propeller have to be in use?  A motor glider with the engine off and propeller retracted would be quieter than a human powered aircraft with the propeller turning.

Are you asking about the noise inside the aircraft or the noise on the ground?

@AnonymousPhysicist yeah, that would desqualify it, it wouldn't be fair to compare it to others either.

@AnonymousPhysicist outside, people hearing it from outside...updated post.

No solid noise data available, therefore only as a comment and not an answer, but from the design those two should be contenders: Solar Stratos and HY4.

@PeterKämpf Although solar and fuel cell electric aircraft will have essentially no engine / motor noise (though electric motors can have a high pitch whine from their inverter), unless the system was specifically designed to be quiet, the propeller will be a significant source of noise.  Efficient, lightweight props have heavy blade loading and have high tip speeds -- quiet props have light blade loading and tip speeds as low as possible.

Although it was never commercially available, the Lockheed YO-3 Quiet Star was a Vietnam observation aircraft designed to fly at 1000 ft agl undetected at night.
It was based on the Schweizer SGS 2-32 glider and used a Continental IO-360D.  Later versions used a constant speed wooden propeller.  It was equipped with an elaborate exhaust system to help silence engine noise.

So quiet, it was used as a noise measurement platform for other aircraft

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can the V22 Osprey glide after dual engine failure while cruising?
In the event of complete power failure, if the aircraft is at cruise phase having transitioned to horizontal flight, can it glide and land?
If so, how does it deals with the propellers not having enough ground clearance while in  horizontal mode?

As I recall, the rotors can be feathered, similar to the propellers on any other multi engine airplane. Upon impact with an object, the blades are designed to be frangible and shatter into strand like sections.  Think of cooked spaghetti attached to the hub.

oh ok, so there would be indeed a prop strike

Only partially answered in Can the V-22 autorotate?: "if required, the V-22 can glide for a predictable run-on landing in airplane mode, much like a turboprop" .

Even a rock has a measurable glide ratio of <some_laughable_fraction>:1. The situation is similar with the Osprey. It's not designed to "glide" at anything remotely like a usable ratio, so instead it is designed to be survivable on impacts that would shred most other aircraft.


text:
Was the Boeing 2707 design flawed?
The mach 2.7 capable Boeing 2707 has an interesting design. I'm curious if there has been any studies done that concluded that this aircraft would simply not have worked, given the way it was initially designed, even if sufficient funding existed.
If so, what was the problem with the design? By problems, I don't mean it being financially feasible or sustainable, I mean technical limitations and restrictions imposed by physics. Could this sort of aircraft ever have achieved a sustained mach 2.7, and carry 277 passengers?
I'm personally inclined to believe it might have worked, considering other notable aircraft of the time, such as the military planes; the XB-70, the SR-71, or the civilian Concorde, and Tu-144. The civilian planes mentioned here are somewhat slower, but the military planes are somewhat faster, and puts the 2707 right in the middle.
However, it would be nice to see an actual evaluation of its design, if such a thing exists.

The three competing supersonic designs are discussed in this video – The Forgotten American Concordes. Two competing designs from the USA were proposed to the US government, which was going to fund 75% of the development cost: one from Lockheed, and the B2707 from Boeing which was the winning design.
The B2707 was meant to be bigger and faster than Concorde, and this was one of the three issues that eventually saw the project being cancelled:

Concorde flew at Mach 2, heating up the nose to 127 °C and the wing leading edges to 100 °C. The whole plane was considerably longer at cruise than it was on the runway due to heating up from the supersonic flow, but at this speed aluminium could still be used as the main construction material for most of aeroplane. The B2707 would be heating up much more, ruling out aluminium and having to use titanium, skyrocketing the construction price per aeroplane.
The oil crisis of the early 70s raised fuel prices a lot, with unfavourable consequences for the operating costs. It was thought that since the travel time is shorter there could be more daily flights, but:
protests about supersonic booms prohibited supersonic flights over most land, meaning that the only viable market was the US east coast to the European west coast.

If so, what was the problem with the design? By problems, I don't mean it being financially feasible or sustainable, I mean technical limitations and restrictions imposed by physics. Could this sort of aircraft ever have achieved a sustained mach 2.7, and carry 277 passengers?

Image source
It might have, there were military aeroplanes flying at M 2.7 or faster at the time like the XB-70 Valkyrie built out of stainless steel, but the technology was still immature. Passenger transport implies high reliability and long testing times to verify if the safety requirements can be met, and this was a major issue to be crossed out at the time of cancellation.
So the question: "Could this sort of aircraft ever have achieved a sustained mach 2.7" at the required reliability rate was never tested.

There is also now a US-Asia market, which has unbearably long flights as things are right now, and close to 100% of those flights are over water.  This would help Emirates and the middle eastern carriers a great deal less, as most of their flights are mostly over land, where the booms would be not appreciated.  That would greatly aid US carriers, for instance Singapore-London would work best as supersonic to SFO, subsonic to Dulles then supersonic to the Irish Sea.

@Harper It sure looks like a different market nowadays, Japan to California for instance.

Civilian airlines also have an FAA requirement to descending down to non-pressurized cabin altitude within a set amount of time.  This was a huge problem for the concord during design, because it means the aircraft needs to do a much more significant altitude drop in the same period due to their higher cruise altitude.

@Harper you can't be serious about the Singapore route. http://www.gcmap.com/mapui?P=sin-sfo-iad-lhr&MP=polar&DU=mi

@DouglasHeld Holy smoke, that was dumb of me. I blame Mercator!  But yes, that is a problem, too many of the world's routes are heavily over land... That's just not an American problem which is why an SST would be so awesome for us...

@DouglasHeld: On the other hand, Singapore to London via Beijing would have the vast majority of the PEK-LHR segment passing over sparsely-populated parts of Mongolia, Siberia, and the Russian Arctic (probably doable), and yet be only 16% longer than a direct route, while SIN-TRV-BCN-LHR does even better, being only 13.7% longer than a direct SIN-LHR flight and yet being mostly over water (and slight routing adjustments could improve its overwater percentage further).

Also, IIRC, the 2707 was to have been built mainly from stainless steel, not titanium.

The 2707 started with a variable sweep, but it was too heavy to be feasible
Here's a mockup of the 2707 with its variable sweep

And here's a video of Boeing's mockup

But variable sweep mechanisms add a lot of weight. And, being a civilian airliner, there would likely have to be all sorts of safety backups, adding more weight. As the video notes, the 2707

could cross the Atlantic, but without passengers

And Wikipedia notes this quote

Boeing also faced insurmountable weight problems due to the swing-wing mechanism (a titanium pivot section having been fabricated with a weight of 4,600 pounds and measuring eleven feet long and 2.5 feet thick)[15] and the design could not achieve sufficient range.

The redesigned 2707 went with a delta wing like the Concorde, but by then other cost factors caused the US government and Boeing to cancel the project

Note the larger tail, the placement of the engines, the canard.  Should have been the 2737.

The wings could be folded for storage like the Handley Page type W, would not have to swing in the air (keep it subsonic).

Was the redesign with a delta wing also slanted for mach 2.7?

@AlphaCentauri I can't find anything that says the new design was any different in speed from the old

Even after all these years, I can recognize a KSP render when I see one :)

I wonder if that wing is more feasible as a design now with modern materials/manufacturing

It certainly would have worked, as there weren't really any technical obstacles, only financial.
The real issue is, was it possible to build it and put it into service with un-subsidized ticket prices that would fill seats AND make it possible to make a profit on its operation?
It's important to note that the Concorde was only feasible because British and French government direct operating subsidies made it possible to charge only astronomical prices for tickets, vs down-a-black-hole prices that would have been required without subsidies (even with subsidies, traveling on Concorde was about as expensive as traveling to Europe on a Clipper flying boat in the late 30s.  The British and French governments were basically subsidizing, or giving away money to, rich folks for national pride reasons).
Even allowing for the fact that Boeing benefits from de-facto subsidies in the form of military development contracts for designs with ultimate civilian use, and assuming such had been available for the 2707 (a military related development program to get it off the ground that is), the direct operating costs would have made it unfeasible in a non-subsidized airline environment (if the KC135/707 wasn't able to make money in 1958 at the airline operating level, it wouldn't have worked either).
Bottom line: the operating numbers simply didn't work out, except as a  "national pride" sort of project, like Concorde, and outside of military applications, that sort of thing doesn't happen very often in the US.

Sounds like a lengthy personal opinion. Developing any airliner costs a lot of money. Supersonic travel was economically competitive with the turbojets of the time, before high bypass turbofans were developed.

3/4s of what's on here is "informed opinion".  If everything required hard data included in responses, why bother with questions... just look it up.

Why would you want to be just another person who says something on the internet?

Can you speak as to how a privatised British Airways made Concorde profitable in the 1980s, without any subsidies and while paying the British government a profit share?

Both operations (British Airways and Air France) were heavily subsidized.
In the case of Air France, the state granted funds directly, while to British Airways, the state granted loans, and some loans have not been paid.
So, we have two governments helping it.

you're quite wrong saying the problems were purely financial. The design was (in part because of the requirements for it) overly ambitious and dependent on technology that was immature at best, non-existent at the time at worst. The work towards building the aircraft did help the aircraft industry a lot for the future, but it's highly unlikely it would have yielded a functional aircraft meeting the design specs, certainly within the time constraints put forward by congress.

@magallanes do you have any supporting evidence that British Airways has unpaid loans related to Concorde?  There's a lot of crap surrounding Concorde, a lot of it false.

I don't think people are mainly complaining about the lack of evidence for the financial aspect. What sets people off is this: "there weren't really any technical obstacles". The fact that financial problems were (apparently) insurmountable just means we don't know about the technical aspect. It doesn't mean that the design was technically feasible.

You could google for British airways and their debts.

You could google for British airways and their debts before and after privatization (including foreign debts to save money. sheesh!).   But you don't need to go deeper in its finances.  It's common sense.  You won't kill a product that generates incomes.  The 737 is older and is still active.  France was frank: it was a waste.

@magallanes I've done a hell of a lot of research on Concorde in BA service in the past, and I can safely say that I don't need to do any more in order to call your comments rubbish.  There is no argument that BA was subsidised (it was a state owned and run airline, of course it was subsidised), but that's not an argument against Concorde.  BA paid in full for the Concordes it ordered, received a couple extra at discounted rates after foreign orders were cancelled, and ran those Concordes profitably after privatisation in the 1980s.

@magallanes and then there's always the argument that even if Concorde was subsidised, at least Britain and France got an operational aircraft for its money - the US tax payer got a cancelled mockup.  Or were you unaware that the US SST project was also subsidised and paid for with taxpayer money?

I know the airplane was re-designed during development due to weight and complexity issues associated with the swing wing design.  Variable geometry was the rage in the sixties and Boeing felt that, from initial studies of the problem, that is could make use of the swing wing design for improved low speed flight.  It turned out that even with a titanium wing box, the airplane was just too heavy to carry the proposed passenger load.  A delta wing configuration was selected in later iterations of the 2707 design.  This, combined with the unprofitably, environmental issues, heat control, drove the company to cancel the nearest thing America ever got to an SST.


text:
Can someone identify this unusual plane at SeaTac airport?
I saw this plane at the SeaTac Airport and thought it was very unusual. The wings look too small to even be functional to me. What kind of plane is this and what is it used for?

That's my plane, look you can see me in the cockpit.  When the ceiling is too low to fly I can fold the wings and drive down the highway!

It's the little-known passenger version of the F-104 Starfighter. That thing sticking out of the front is a landing skid.

A rare variant without the AWACS dome.  Had issues with fine yaw control authority.

There is a big clue right underneath the "Sea-Tac Airport" decal - it says

FIRE RESCUE

It can be seen a little better on this image

source: wikimedia commons
It is a rig used for fire and evacuation training.

A unique tool the department uses is an aircraft mock-up designed for aircraft approach, entry and tactics training on the airfield. The smoke generated is not harmful to the environment and provides a realistic challenge.
  source: https://www.portseattle.org/page/training

The image below appears to show it in use:

ie : It's not an airplane at all.

@J...  yes it is -- just one that can't fly.  Ya know, like the  emu of aircraft :-)

a vehicle designed for air travel that has wings and one or more engines - so strictly speaking, it is not an aeroplane.

"big clue" I can read it now I know what it's supposed to say, but otherwise... I'll get more coffee.

is it an abstract mockup, or did they make it from an old aircraft that would have otherwise been scrapped?

@DanNeely looks like a mockup to me. The engine fans dont look real, doesnt look an awful lot like a current or past air frame AFAIK. I might be wrong though,

This is a complete guess, but I'd imagine that some parts will have come from scrapped aircraft and others will have been manufactured to make it look a little more real. Looks like the centre of the fuselage (including doors and windows), the inner part of the wing and the engine shells are real, while the fuselage nose and tail, outer wings, engine pylons and fans are fake.

Better pics:  https://twitter.com/seatacairport/status/857633066386243584?lang=en
https://upload.wikimedia.org/wikipedia/commons/f/fb/SeaTac_fire_rescue_plane_01.jpg

In what sense is this a "unique tool"? Most airports have a plane or mock-up plane used to train fire crews, don't they?

@DavidRicherby, that page is written like a promo for their department.  I'd say unique in the sense that any non-airport fire department wouldn't have that tool.  Perhaps it's "the only one in the area".

@DavidRicherby Or maybe most airports can affort to use an actual plane filled with fuel instead of using a cheap mock up...

So why the heck can the (mockup) wings fold??

@Fattie so there are wings in the way when doing training, but it doesn't take up as much space the rest of the time.

smart, @DanNeely !!!!

@Dan Neely Perhaps also the wings fold so that it could be transported by trailer from its manufacturer to the customer (Sea Tac airport).  Unlikely that this was manufactured on site...

According to this forum post about a Funny Looking SEATAC Plane it's a (fake) firefighter training plane.
Question:

On 9/30 taxing north from SEA to ORD there was a
  funny looking tri-jet plane marked with Seattle
  Tacoma Airport hanging out by the Alaska Airlines
  cargo area. Wings folded up. Anyone know anything
  about this plane or its purpose?

Answer:

Yeah, it's the firefighter training plane .
  You hear pilots asking ATC what it is occasionally.
Here's a picture I found on flickr.
  http://www.flickr.com/photos/smartjunco/331183916/


text:
What were the typical rotation and landing speeds of the X-29?
Looking online through all the available literature, I could not find the typical rotation ($V_r$), landing or even stall speed of the experimental forward swept wing X-29 aircraft.
I'm curious to know if the forward swept wing lowered the rotation and landing speeds over a standard F-5 on which the X-29 was based.
Does anyone know?

This might be hard to answer, since the aircraft never went into service, so it's unlikely a detailed POH was written based on its <300 flights. Also, the wing would not directly lower said speeds, but it would increase the maneuverability at low speeds and indirectly allow slower flight by making it safer to be slow and low.

The wing area may have been different as well.


text:
Why top mounted propellers aren't more common?
If having a longer propeller increases efficiency(lower disk loading), why aren't there more GA airplanes with top mounted propellers like the Chotia Weedhopper ultralight, thus enabling the increase of prop diameter while keeping ground clearance, but Instead the norm is a middle mounted prop like the Cessna 172.

source

A high propeller location can usually be found in seaplanes. Examples range from the Dornier Libelle to the Canadair CL-415.

Dornier Libelle or Dornier A (picture source)

Canadair CL-415 (picture source)
The obvious reason is to keep the propeller away from spray water, but another reason becomes only obvious when we look what happened to landplanes with high mounted engines. One example is mentioned in Bill Gunston's book "Back to the Drawing Board", which presents a range of unfortunate design mistakes. The one with the high engines is the series of Blériot 67 to 75, a series of four-engined biplane bombers from 1916 to 1919 with two of their four engines mounted in the upper wing.

Blériot 73 (picture source)
The Blériot company even patented this engine arrangement since it minimized the yawing moment resulting from engine failure. However, it soon became apparent that for take-off only the lower engines could be run at full power - adding full power to the upper engines would had tipped the airplane over.
Seaplanes with their long hull offer much better pitch stability during their ground run than tailwheel designs, so only they could afford a high engine position. Both still suffer from severe pitch trim changes with power changes, requiring more attention from the pilot and a larger horizontal tail.
Still, your reasoning of using a larger propeller for better efficiency is sound. What has been done to integrate large propellers can be seen in the late WW I period, when increasing engine power made the gyro effects of rotary engines so severe that geared engines were developed where the cylinders rotate in one direction and the propeller in the opposite direction. As a consequence, the propeller had only half the RPM in air as it had with the cylinder block. This gave great propeller efficiency, but also big propeller diameters, so airplanes with those engines needed a high landing gear. Below is a picture of a Roland D XVI with a Siemens & Halske III counter-rotating rotary engine from 1918 (source). This was an excellent fighter aircraft for its time with almost no gyro coupling.

Now the engine is in line with the longitudinal axis of the airplane, but still way above the ground, so the resulting pitching moment makes ground handling tricky. Pilots preferred smaller props with less sensitive pitch reactions to throttle changes, and this carried over to modern tricycle designs which, especially when using ungeared engines, use smaller, faster spinning props.
A large prop will need a reduction gear and a higher landing gear, which both add weight and cost. Why you want a high-revving engine in airplanes is explained here, so low RPM engines don't qualify. The compromise is to use an ungeared engine and suffer the lower efficiency of the smaller propeller, but benefit from the lower mechanical complexity and weight of an ungeared engine.

interesting, your last point about the need for gear reduction is a good one nobody has motioned

Excellent, I'd love to see a jungmeister or yak55 powered by a modern variant of that contra rotary engine

@user721108 Both the Bücker 133 and the Yak 55 use radial engines. The ones with the severe gyro effects are called rotary engines.

ok my bad i misunderstood what's counter rotating in S&H III. prop and cylinders actually spin in the same direction

(i thought prop and cyl spun in opposite directions in S&H III, that could have make sense, and improve cooling btw)

@PeterKämpf  the propeller could have been attached to the crankshaft instead of the crankcase. rpm would still have been halved, but cooling would have been improved by taking advantage of the relative wind opposition between the direction of rotation of the cooling fins and that of the propeller blades. no idea where on SE to ask about that, nor if relevant at all.

@user721108 The invention of the NACA cowling has made rotating the cylinders for cooling superfluous. Only if you have several rows of cylinders AND poor internal aerodynamics will cooling be an issue. The cowling of the Roland D XVI anticipated the NACA cowling. Regarding the S&H III, Wikipedia has an animation showing how it worked.

The aeroplane is easiest to control if the thrust line intersects with the Centre of Gravity (CoG).

The propeller thrust in the ultralight is above the CoG (my estimation of it), and any change in thrust will create a pitching moment which will need to be compensated for with the pitch stick. Not much of an issue normally, except in situations like engine failure during takeoff etc. which will need to be failure analysed for passenger planes.
In the pictured ultralight the propeller slipstream is over the wing area, which helps in creating lift. So yeah, having the propeller in front of a high wing is a no-brainer actually.

Not quite. The aircraft is easiest to control if the thrust line intersects with the center of effective drag, not the COG. (I use easiest to control in the same sense that you meant.)

@A.I.Breveleri https://aviation.stackexchange.com/a/65897/21091

Think of this in terms of a vector sum of thrust, weight, and drag.
At zero airspeed (like when starting a takeoff roll) the thrust of a top-mounted prop causes a nose-down pitching moment that the pilot has to trim away in order to get the AoA correct for best takeoff performance.
Then, as speed builds up, drag is nonzero and the center of pressure of the fuselage starts to add to the pitch-down condition, requiring more trimming.
When the plane leaves the ground, the pitching moment gets shifted  depending on exactly where the center of lift is relative to the center of gravity.
In a perfect world, you'd have the thrust line go through a point as close as possible to the CG, CoL and CoP (and have these locations be as close to one another as possible) so that during flight, you don't need to adjust pitch trim with changes in power setting- and at takeoff, you advance power and the plane naturally flies itself off the runway when the airspeed is right.

There are a few safety issues with this design:

If the engine is in the position like this ultralight it prevents the pilot from having any visibility above and ahead of the aircraft. This makes it much harder to see and avoid air traffic. In a more standard configuration you look over the engine, which blocks your view ahead and below to a certain extent, however when you are turning you can see what you are turning into, which is much safer
With a high mounted engine the thrust is always pushing the nose down, so if the engine quits the aircraft will have a tendency to pitch up. In a climb you'd need very quick reactions to get the nose down or you'd risk a stall

Regarding point 2. Does increasing the prop diameter helps alleviate this issue?


text:
Is a 3 blade propeller at same RPM and diameter quieter than 2 blade?
READ: Knowing that a 3 blade will be quieter for the same thrust compared to a 2 blade as mentioned in 3 vs 2 prop post and multiple other articles(because the size and speed on the 3 blade can be reduced), however my question is, if both are doing the same RPM and the same diameter(hence generating different thrusts), will the 3 blade still be quieter?
A link to a video with a test with a sound level meter would be awesome(I couldn't find any online). Can't believe no one has a video of this somewhere.

No.  Each blade sees more wake from its predecessor, because its predecessor is closer.  So each blade sees more turbulence and thus makes more noise than the equivalent 2-blade, not less.  See e.g. section 3 of JPL TR 32-7462.
That paper cites NACA TN 2968, A review of aerodynamic noise from propellers, rotors, and lift fans, which plots noise vs. blade count (e.g., fig. 2), showing that fewer blades are noisier -- but for the same horsepower (and thus different diameters), unlike this question (same diameter, thus different horsepower).

Still, for the same thrust the lift coefficient will be lower for the four blade prop. This means all pressure variations should be lower, too. Lower overspeeds also means lower tip Mach peaks.

+1 for the JPL paper. It says on page 6: "Hubbard, in Ref. 8, constructed, from solutions to this equation, plots which show that the noise level increases with absorbed power, increased diameter, fewer blades, and especially with increased tip speed".

Most 3 blade conversions of 2 blade engines, very popular on the DeHavilland Beaver, seem to be quieter only because the blades can be shorter to absorb the torque and don't become transonic.

I'm confused, if the noise level increases the fewer blades it has then that means that the 2 blades will be noisier than 3. Is this answer wrong then?

No contradiction.  That paper says that fewer blades are noisier when power stays constant.  I've edited to clarify.

So in short, if the blade length is the same, 3 blades is noisier than 2 at any given RPM, but 3 blades produce more thrust, so in practice the 3 blade prop is quieter because the blades can be shorter and run at a lower RPM in any given flight regime.  Is that about right?

Honestly I find this study confused and overcomplicated for my question and in the wording and so old, I can't believe there isn't a video out there someone has done this test....this is not like a million dollar study to do by NASA, its just put two props and see which one makes more noise at same RPM with a sound level meter.  My question is very simple.

@DarthPseudonym, right.  Design in practice doesn't add or remove blades while keeping rpm and diameter constant.  It also varies those.

At the same rpm and diameter, more blades aren't quieter, they're louder.
My other answer shows that, as blades are added, each blade becomes louder.  But it's even simpler to show that, as they're added, the propeller as a unit becomes louder.

The noise comes from the air disturbed by the propeller.

This disturbance is overwhelmingly from each blade.

So, at fixed rpm and diameter, more blades means more disturbance, which means more noise.  QED.

What's hard about recording a video demonstration of this is simply finding two otherwise identical props with different blade counts.  An inexpensive way would be to buy two 4-blade model airplane props and from one of those cut off two opposing blades to make a 2-blade.  Or find a hub with room for either one or two 2-blade props, to create a 4-blade.
Finding a matching 2- and 3-blade is even harder.  One vendor has a pretty close match, but even there some dimensions slightly differ.


text:
Is there any type of mechanical engine failure that would inhibit a helicopter from making an autorotation?
My understanding is that the motor shaft is connected to the rotor using some sort of clutch so that if the engine shuts down the helicopter rotor can keep spinning.
However I've heard of some fatal accidents with helicopters that happen after some sort of mechanical failure in the engine(like something breaks or something).
If someone can clarify this, my question is, is there in fact any type of failure in the engine that wouldn't allow the pilot to make an autorotation?

I could be totally wrong here, but I believe you need a certain minimum altitude to effectively autorotate.  If you experience an engine problem with insufficient altitude you  may not have enough time to auto rotate and wind up with a fatal crash as a result.

@ForwardEd   I'm not an expert but have looked at autorotation initiation. It needs to be initiated in the range 2-6 seconds MAXIMUM with this figure varying by aircraft. The Robinson R22 is one of the shortest. ASAP,  and under 2 seconds would be a VERY good idea. || The time avaiable is how long the rotor still spins with enough energy to pick up velocity again when pitch is fully reversed. If you leave it too long the rotor will not spin up and you fgall :-( . || Once you have changed pitch you are in autorotation. How long it takes for full spinup I know not. BUT see next comment ...

@ForwardEd What I said was (more or less) right BUT effectively misleading. The link below shows a safe height:velocity diagram for a Bell 204B. Aty zero forward velocity they recommend 800 feet altitude for entering safe autorotation, but at eg 55 knots its 75 feet. 204b: https://wiki2.org/en/Height-velocity_diagram || Robinson R22 video: https://www.youtube.com/watch?v=grvy3ft6uFI&t=1s

Some constraints are taken into account when designing the rotor transmission:

The clutch used to drive the rotor must be free-wheeling when the engine is malfunctioning and/or the autorotation is able to drive the rotor faster than the failed engine.
The clutch is a one-way coupling disengaging in case of engine failure.

The anti-torque rotor is directly coupled to the main rotor. Autorotation doesn't produce much torque but the anti-torque rotor is used to effectively yaw the helicopter.

Some helicopters are also equipped with a clutch preventing a slow engine to continue driving the rotor at a slow speed. This is to prevent the rotor to completely dissipates its kinetic energy before the pilot reacts to the engine failure and lowers the collective pitch to initiate the descent in autorotation.
Autorotation clutch: Relative difference of speeds

The autorotation clutch or sprag clutch only transmits drive one way. The dog bone-shaped elements
twist and lock between the inner and outer races to transmit the drive. If the relative drive direction reverses, the dog bones turn the other way and slip. Source: The Art of the Helicopter

Similarly on a twin-turbine helicopter, the coupling of the two engines is designed in a way one engine cannot prevent the other to drive the transmission alone. Each engine has its own one-way clutch:

Source: Ibid.
The only possibility for the rotors to be prevented to spin is the shaft/mast are jammed or broken, a possibility made unlikely by their sizes and their designs.
Centrifugal clutch: Fixed speed
In addition of the autorotation clutch(es), some helicopters have a centrifugal clutch preventing the rotor to be driven by the engine at low rpm. This clutch disconnects the main rotor shaft from the engine(s) as soon as the engine(s) is (are) not turning fast enough. This prevents the rotor losing its kinetic energy, the latter being helpful, sometimes necessary, to start the autorotation.
While the autorotation clutch disconnects the rotor as soon as the engine spins more slowly than the rotor (difference in speeds), the centrifugal clutch disconnects the rotor as soon as the rotor speed is below a fixed threshold, regardless of the engine speed.
See What is the purpose of a freewheeling unit in an helicopter, if a centrifugal clutch is already present?

In the crash of the second pre-production EH101, the failure mode did prevent autorotation. Not actually an engine problem, though
The control rods were destroyed by a fire started by frictional heat generated by a rotor brake problem.
https://web.archive.org/web/20120308214147/http://www.flightglobal.com/pdfarchive/view/1993/1993%20-%201144.html

That would have been a nasty feeling. (There must be a better way to put that :-( ). One can perhaps sort of accept accidents where you are at fault or another 'vehicle' is*, but having yuour machine just "stop flying properly" is not on.  || * I've been in two car vs motorbike accidents where the cars were at fault. Neither was very welcome, but on a motorbike it's an accepted risk.


text:
Can you fly a newly EASA type certified aircraft in the USA?
If you get an aircraft that has just been issued a type certificate by EASA, can you fly it within USA in the same category right away? I'm reading about Bilateral Agreements and it seems that the answer would be yes, but I'm confused because for example the Pipistrel Velis Electro was certified more than 4 years ago by EASA yet it doesn't have the same category(only as experimental) in the USA.
How does this process really works?
What's the point of having a bilateral agreement? (it seems as if the FAA has to re-work everything EASA already did)

A starting point for an answer: EU - USA Bilateral Agreement

I can't find the answer to the question in that link...

Type certificates go through an easier process of recognition but there is no automatic recognition.

There are several separate issues here, and this will be lengthy but only partial answer to your question.
A type certificate issued in any ICAO country permits flying into and within any ICAO country. So an aircraft that has type certificate in EASA can fly into USA right away - but it cannot be registered in USA before FAA grants an American type certificate. So an European airline can acquire a brand new EASA certified aircraft type and begin to operate to USA, but a US airline has to wait until the said aircraft type is type certified by FAA until beginning to operate it.
An aircraft cannot be registered in a country that does not recognize the type certificate granted by another authority. This issue has been prominent in Europe with aircraft from former Soviet countries. For example, many Eastern European countries have historically approved Russian type certificates. That means several Russian built aircrafts exist and are registered into these countries. Even though they are now EASA countries, the aircrafts' registrations cannot be transferred under other aviation authorities as these nations have not approved Russian type certificates - even within EASA. This also prevents aircrafts only certified in Europe to be registered in USA.
Commerial operators have on top of that additional international hurdles. Airlines operators certificate granted by the local authority dictates where the aircrafts' operated by the airline have to be registered. As a general rule, an airline cannot acquire an aircraft registered in another region and begin to operate it at different area. So an US airline cannot by default operate an aircraft that has been registered in Europe. Some aviation authorities allow exemptions on this, but that is up the local authorities.
Finally there is the matter of traffic rights. These are called "Freedoms of the Air". By default, the most basic of them such as overflying another country and a right to land for a technical stop are in force between all ICAO nations.
There are nine of them, and it is up to the nations to agree which ones are applied between them. These are called Bilateral agreements.
You can view all the freedoms here: https://www.icao.int/Pages/freedomsAir.aspx
But basically, to obtain the type certificate in EU, USA, China etc. the manufacturer has to repeat the entire process with the said authorities - at least on paper. Some type certificates may even require addition test flights under supervision of the said authority. In practice, especially FAA and EASA type certificate requirements are pretty well harmonized but not identical. But maybe somebody else can shed light on the actual process (and pricing!)

So, that means according to what you say and this https://aviation.stackexchange.com/a/95669 that I can purchase and register the aircraft in Europe, fly it to USA, and simply fly it indefinitely in USA just renewing my EU registration whenever needed.


text:
What are the most common types of FOD (Foreign Object Debris)?
Would like to know what are the 2 most common types of FOD.
More specifically would like to know which ones causes the most incidents/accidents.

I think the answer to this question varies quite a bit from airport to airport and area to area. Runway FOD is usually of different nature (stones, dead animals) than Apron FOD. Of course one could aggregate all FOD data worldwide, but what would the benefit be?

I understand your point. I'm just curious, actually more specifically which FOD causes the most incidents/accidents. Updating post.

I think the answer to this question varies quite a bit from airport to airport (and aircraft carriers) and from operation area to operation area. Runway FOD is will be of different nature (rubber chunks, stones, dead animals ) than Apron FOD (plastic bags, luggage tags, soda cans).
The following is quoted from the FAA's Advisory Circular AC-150-5210-24

c. FOD Taxonomy.  The exact nature of FOD is also varied.  FOD can be composed of any material and can be of any color and size.  In a one year airport study (Information Paper on French Study on Automatic FOD Detection Systems – Workshop EUROCONTROL, 9-10 June 2008), over 60% of the collected, known FOD items were made of metal, followed by 18% of the items being made of rubber.  Dark-colored items made up nearly 50% of the FOD collected.  Typical FOD includes the following:

aircraft and engine fasteners (nuts, bolts, washers, safety wire, etc.);
aircraft parts (fuel caps, landing gear fragments, oil sticks, metal sheets, trapdoors, and tire fragments);
mechanics' tools;
catering supplies;
flight line items (nails, personnel badges, pens, pencils, luggage tags, soda cans, etc.);
apron items (paper and plastic debris from catering and freight pallets, luggage parts, and debris from ramp equipment);
runway and taxiway materials (concrete and asphalt chunks, rubber joint materials, and paint chips);
construction debris (pieces of wood, stones, fasteners and miscellaneous metal objects);
plastic and/or polyethylene materials; - natural materials (plant fragments, wildlife and volcanic ash); and
contaminants from winter conditions (snow, ice).

FOD removal operations are not meant to occur when a given area is contaminated with snow or ice.  In such winter conditions, the procedures listed in AC 150/5200-30, Airport Winter Safety and Operations, are used to clear the AOA surfaces.

Around the ramp area near the gates, it's zipper tags everywhere.

One possible answer is that the most common FOD is unknown! A 1998 study on US Air Force FOD prevention practices and procedures included a review of DoD statistics on sources and root causes, in conjunction with the rest of the study. What they found is probably similar to the civilian world (page 3, emphasis mine):

For the study, data on DoD FOD incidents was obtained from the Air Force Safety Center and the Naval Safety Center. This data, while useful in providing a background on the types of FOD incidents, did not lend itself to extensive statistical analysis. There were three reasons for this: 1) Some of the information held by these organizations is classified and cannot be released, which led to an incomplete data set for analysis. 2) Different military units collect, track and report different types of information. What is considered a FOD incident in the Air Force's Air Combat Command is not necessarily the same thing as a FOD incident in the Navy, or even in other Air Force Major Commands. 3) While all FOD incidents are investigated, the root cause of the incident is never determined in many cases (almost 50%)."

If half of the incidents didn't have a root cause identified, a source of FOD which is harder to identify in an incident will likely be deeply undercounted, making the tallying up of the remainder a potentially poor guide to what, in total, was out there causing problems.


text:
Has a carbon fiber propeller ever broke with a bird strike?
Curious if a carbon fiber prop has broken(or considerably deformed) with a bird strike. Or maybe by something else other than an impact with the ground/crash.

I don't believe that the response to a prop strike (caused by a bird or anything else) would be different for a composite versus another material.
The company which manufactured one of the first composite blades has an info page which is primarily concerned with damage to what's attached to the propeller. Their guidance is to perform a complete inspection of the engine to ensure that other failures haven't been caused by the strike.


text:
Which FAA regulations apply to the Stratolaunch?
(Image source)
Which FAA regulations apply to the Stratolaunch being built by Vulcan Aerospace and Scaled Composites?
Do they follow the same regulations for the type cert, production and CofA (Certificate of Airworthiness) just like a commercial airplane built by Boeing?

That would be under Title 14 CFR, Chapters 400-460.
A lot of this stuff really hasn't been finalized yet and these vehicles are considered only experimental and regulated similar to homebuilt aircraft.  Passengers are required to sign waivers advising them of their risk and that they understand this and hold the organization indemniable to any and all liabilities involved with using this system.

400-600 apply to the space vehicle itself, the carrier aircraft probably falls under a different set of CFR's, probably the experimental category.

Well in the case of StratoLaunch it is unmanned - or at least the booster is.  One could consider the system a rocket launch from a flying launch pad.  In Re for the carrier aircraft, most likely considered an experimental category vehicle.

@ymb1 It's not a link-only answer (and wasn't at the time you made your most recent comment).

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How much difference in drag there is between a feathered and non-feathered airplane?
Considering a typical twin like a Beechcraft Baron/Piper Navajo, if you lose both engines and no feathering is done how much drag will you have compared to if feathering was done.
Or in another way, what I really want to know is a rough estimate, about how much (percent wise) will your stall speed increase in a non feathered vs feathered scenario?

One does not have a feathered airplane; one may have a feathered propeller.

A feathered airplane is what we call a "bird".

The increase in drag is quite substantial.  The non-feathered blades continue to spin.  They are connected to the in-operative engine, so the engine keeps turning over.  You continue compressing air like a pump.  The power to keep the pump pumping is taken from your kinetic and potential energy.
All that said, this does not change your stall speed.  There may be a small difference in power on vs. power off stall speed, but this is due to the propeller blowing air (or not) over the wings.
The big difference you will see is in your power off rate of descent.  The increased drag will cause an increased rate of descent -- at whatever speed you're at.

ok. Interesting point about the engine. So, does that means that if the propeller wasn't attached to an engine(just imagine an hypothetical airplane with the propeller unattached to the engine) would you be able to glide a longer distance?

Yes, if you had a propeller that was just free-wheeling, it would extract less power, cause less drag, and you would glide longer.

I'm not sure how much energy the compression is really going to steal. If you don't have a detonation increasing the pressure in the cylinder to add energy to the system or a jake-brake releasing the compressed air to remove energy from the system, the air trapped in the cylinder is just going to act like a spring and push the energy back into the prop on the next stroke. You'll lose some energy to friction but I don't think the compression is going to make you lose any more energy than if you just had the valves all open all the time.

@DarthPseudonym The mechanics of the engine -- crank, cam, valves, etc keep working.  In this mode, the engine compresses a slug of air each stroke and discharges it.  If you've ever turned an engine over by hand -- and then removed the plugs and done it again, you'll realize that the former is much harder.  In many ways, you are engine braking -- but without a dedicated exhaust restriction.

In terms of piloting with a failed engine -- best (least drag) case, feather the prop, it comes to rest.  Worst case, feathering does not work, engine windmills with engine acting as a load.  Intermediate case -- feathering does not work, engine seizes.  The seized engine case actually has less drag than the windmilling case.

@RobMcDonald No, that's what I'm saying. You're only engine braking if you open the exhaust valve and release the compressed air with a pop while the cylinder head is up. That's what a jake brake is. If you don't do that, then all the energy you spent compressing the air is still in the cylinder and will release by making the expansion stroke easier -- the effect is the same as if you put a large spring in the cylinder instead of air. You can't ever recover ALL the energy (thanks, thermodynamics) but the loss from compressing and expanding a sealed container is pretty small.

@DarthPseudonym When I drive my light pickup truck (no Jake Brake, but with a manual transmission), and I down shift and engine brake, I slow down quite rapidly.  Much more rapidly than if I keep the clutch pedal down and coast to a stop.  Even if we ignore the compression / expansion strokes (as a spring), we still have the intake and exhaust strokes that make an engine act like a pump.

Yes, there are more losses when you're using your kinetic energy to move whole load of gears, shafts, and pistons than if you don't do all that. No argument there. I misspoke by saying you were not engine braking if you don't use engine compression braking, I'm only saying the compression of gas in the cylinder isn't a significant energy sink unless you release it to atmosphere while it's under pressure.


text:
Are helicopters required a higher drop/impact test than airplanes?
Considering an airplane and a helicopter both with similar weight and number of seats and both to be certified in the same category.
Are helicopters required a higher standard than airplanes due to  its vertical flight nature or is it actually the opposite or there are no differences?

I think you should see 14 CFR Parts 27 and 29 for helicopters and 14 CFR Parts 23 and 25 for airplanes.


text:
Does setting the parking brake while airborne affects drag?
If you're flying a fixed gear airplane and set the parking brakes while cruising, will this cause an increase or decrease in the drag? Or will this have any other effect on the aircraft?
UPDATE:
After doing more detailed research and watching many GoPro footage people post online from various fixed gear airplanes in all flight phases(takeoff, cruise, landing) I realized that the gears do not spin during cruise on the first place(as mentioned by @abelenky on personal observation)
It seems as if the air speed/friction with the gears is not strong enough to make them spin to start with.
And thinking about it I think it will be very hard to find an airplane where this occurs, since the drag of having fixed gears wont allow it to fly that fast, hence the invention of retractable landing gear airplanes.
Which brings me to this question: What is the fastest fixed gear airplane?
We might be able to find footage of such airplane and see if the phenomena occurs.

One might imagine that a gear wheel with a cowling over the top might be spun up by drag. My intuition says that once fully spun up (with axel friction balancing the drag moment) the overall drag on the vehicle would marginally decrease because the relative velocity would be lower. On the other hand, my intuition also recognizes some analogy between the wheel and a ram air turbine. You could extract work from the spinning wheel, which makes me feel like there could be some energy argument for an increase in required thrust (or more drag). Very interesting question.

From direct, personal observation in a C-172: My main gear does not spin in flight, so setting the parking brake would not change anything. I do not know if this would apply to all aircraft.
The ENORMOUS safety risk is that you could land with the parking brake set, which would be immediately disastrous. No possible fuel savings is worth that risk.

Yes, I'm aware of the risks of doing something like this, the question is only about aerodynamics/physics.

@Gabe You could ask on the Physics, but you will have to explain what parking break is, how it works etc.

Out of curiostity, are GA aircraft tested/certified for this kind of condition (landing with brake set, tire blowing up and consequences?)

@Dimitar, do you really think the people on Physics SE don't know what a parking brake is, or is that a dig?

@MichaelHall, if you don't specify this sort of thing, you'll get answers that only apply to a spherical airplane in a vacuum.

@Mark, you have a point there... ;)

@MichaelHall You can't expect people to know everything about everything.

@Dimitar, right... but if someone is smart enough to answer such a question there's a non-zero chance that they own a car.

If the landing gear is not inside of the aircraft during flight, and the wheels pick up speed (start spinning), then activating the brakes on the wheels will decrease rotational drag caused by the spinning wheels.
On the other hand, if you have your wheels extended, that by itself will drastically increase drag, they really should be tucked into the aircraft for optimal drag performance. Stopping them from spinning is like picking up an elephant with a toothpick compared to the drag caused by the extended wheels.

The question specified "a fixed gear airplane", so retracting the gear isn't an option.


text:
What's the effect on thrust/balance of a quadcopter with propellers with different ground clearance?
In a quad setup like the Air One where back propellers are at different ground clearance than the front propellers, what are the effects of such setup vs if all props had the same ground clearance.

(source)
For the purpose of this question let's asume the back props are at double the front props clearance as shown in the image. Also let's asume the aircraft weight is perfectly balanced.
For a straight vertical takeoff, will back and front props have to spin at different speeds?
Will all props generate the same lift at the same RPM during takeoff?
Does the RPM needs to be adjusted on the front/back propellers to keep the aircraft balanced as the aircraft increases altitude(no ground effect)?
Yes, I know the Air One example technically is not a quad but a Octacopter, just ignore that.

This aircraft sits at a nose down attitude on the ground. It's propellers are clearly tilted forward.
For a straight vertical takeoff, thrust has to be vertical. This means before taking off, it has to rotate while still on the ground, around main landing gear wheels as a pivot point. A reaction force with the ground is needed to prevent it from moving forward. Therefore it needs brakes applied during this phase.
Once at an attitude where propellers' discs are parallel to the ground, it can takeoff vertically like any other quadcopter, adjusting rpm like any other quadcopter, and there is no noticeable ground clearance difference between front and aft propellers anymore.
This video shows how it's performed, and attitude during stationary flight. One can visually notice front propellers increasing rpm relative to aft propellers for that rotation purpose.

(rotation is at 12 seconds, takeoff at 20)

Oh, thanks for the video, so basically it starts by adding more power to the front props, then once the clearance is equal (@ 00:16) all props run at the same power. Interesting.

+1 but there is likely more to explain, as the neutral attitude is not with the propellers at the same height (compare with the horizon)., perhaps more efficient for forward velocity, and the wing providing some lift.

@mins OP is about ground clearance, ground effect (Koyovis answer) and vertical takeoff with no horizontal velocity. Probably the attitude it sits on the ground is for rolling takeoffs with wings creating lift, and wing incidence w.r.t prop disc angle is optimized for lift and drag during cruise . Difference in height while hovering is small, and there's probably smth interesting too regarding that angle between each pair of contrarotating props (those are not exactly coaxial)

Propellers and rotors are subject to ground effect: lift increases when the rotor approaches approximately within 1 diameter distance. This is the reason why a landing helicopter manages its own touchdown to a degree: the closer the rotor approaches the ground, the more vertical speed deceleration.

Pic above is from this answer which describes the rotor ground effect mechanism. The figure is from Principles of Helicopter Aerodynamics by j. Gordon Leishman, the next figure 5.36 shows a quantification of increase in lift in ground effect at constant power, showing about a 10% increase in lift at a distance of 1 rotor diameter.


text:
Are floats considered "landing gear"?
Considering the actual technical definitions, are floats on seaplanes considered "landing gear"?

In what context?  What "technical definition" are you referring to?  (in the context of your other question that spawned this one, consider it or not at your diescretion.)

I mean like regulators FAA, ICAO, EASA, etc definitions

Ok, but again for what context and purpose?  Required maintenance procedures on landing gear during annual and 100 hour inspections?  If they call for putting the airplane on jacks to "swing the gear" or to measure and inspect disc brake pads the obvious answer is no.  If it is a structural specification for the design strength of struts to support the weight of the aircraft, then obviously yes.  You need to specify why you are asking if you hope to get a useful response...

In the context of categorizing airplanes, like retractable and non retractable landing gear airplanes.

They are categorized as "Seaplanes".  (we don't look at a seaplane and wonder what to call the floats, we look at the floats and call it a seaplane.)

I would consider the floats on a floatplane to be landing gear.  For most floatplanes, they are removable / optional and could be replaced with fixed gear instead.
Flying boats have the fuselage as a hull of a boat -- I would have a slightly harder time considering the hull of a flying boat as landing gear.  In particular because many flying boats are actually amphibious -- where we would clearly consider the retractible wheels as landing gear.
There are also floatplanes that have amphibious floats -- floats with wheels.
When I have thought of helicopters with skids, I think of them as landing gear.  Helicopters also can have floats -- which are little more than puffy skids.

"I would consider the floats on a floatplane to be landing gear", is it an opinion or do you refer to some  framework, legal or technical, for which this choice is decisive?

"helicopters with skids" ... That is another good point


text:
Why aren't aircraft built with carbon fiber tubes?
Though carbon fiber is more expensive than aluminum as a base material, one of its biggest costs is creating the final composite parts made of carbon fiber (molding, layering, resin, curing, baking, etc). It is very time consuming and demanding.
As they do with aluminum tubes, why don't aircraft manufacturers simply buy prefabricated carbon fiber tubes (built in large quantities and at a cheaper price), cut them, use joints/unions, and use them in aircraft, especially those with low weight limits like ultralights, Light Sport Aircraft, etc?

(source)

I had a carbon fiber bike once and the frame fractured from riding over a rock now imagine if that same frame got hundreds of pounds of hail and water thrown at it and lightning struck also I don't think that'd be ideal.

Please post the original source of the photos. Thanks!

We'd appreciate it if you could mention the photographer or copyright holder of the image and include a link to the source. See this post on our meta site

could only find one the images...

The second photo seems to be an aluminum t-fitting for use on carbon fiber RC models and drones, made in China and sold on sites like 3DXR and Alibaba. The exact same photo that you are showing can be seen on all of the shopping sites and so the photo was presumably supplied to the sites by the manufacturer.

A few are. The Carbon Corsair is an ultralight built with a fabric-covered frame of CFRP tubes for the fuselage.

source
This is an all-new design. Using CFRP tubes allowed it to have a +6/-4 g limit while fitting into ultralight weight limits. There's even an electric version, which still fits.
As to why CFRP spaceframe construction is rare, compared to semi-monocoque, is that the latter is even better. There, CFRP also adds high stiffness for a load-bearing skin, and the ability to make very complex shapes (at a price).
Early carbon bicycles (and very few modern ones) also used lugged construction. It has been replaced by monocoque frames since. And that's bikes, which only need the frame, not the skin. For aircraft, CFRP semi-monocoque allows for a lot more design flexibility.
Below is one example, the Song Ultralight / ElectraFlyer ULS, which weighs as much as the Carbon Corsair, but uses a much more complex shape. A major advantage of CFRP monocoque over fabric covered is that the body forms a large fuel tank, no need to install separate cells.

source
Other than not sharing the sealed body and shape advantages of semi-monocoque, or not being as cheap as metal tubes, using carbon tubes for the aircraft's spaceframe is a valid technique, though rarely the best one.

If you're going to build from carbon fibre, you generally aren't going to bother with a very "old fashioned" structural method like tube and fabric; you're going to go with the natural advantages of composite construction by building a monocoque structure with carbon cloth and roving instead of fibreglass, and if you're really smart, Graphlite pulltruded carbon rod for key structural elements like spar caps.
There's also a certain amount of hassle in that you need to create joints that have the same strength as the tube, which requires fittings that are a bit more complex than the molded plastic fittings pictured. Bicycle frames do it using layups of carbon tape around joints, but they don't have to cope with the sort of multi-tube joints you have with a 3 dimensional warren truss fuselage. A bit of trouble to design and control, quality wise.
In any case, it's tube and fabric, not considered optimal for modern designs. It's pretty easy to cut and fit steel tubes to come together in a cluster of 5 or 6 tubes all meeting at a single point, and welding the whole mess together. That kind of intersection in CF tube would require an elaborate moulded or machined joint fitting.
You're basically building an 80 year old design, just replacing the steel tubing with carbon. More trouble than it's worth when you can make something more efficiently in a female mold using standard composite methods and have a ready to use part when it cures.

In addition to the other great answers, consider some drawbacks of carbon.
E.g. Fibreglass will be more wear resistant than carbon fibre, which is why you prefer it on leading edges, etc.  (see first picture)
Also the joints you mention are not so trivial, and are likely the weakest parts of the structure. The loads your plane experiences in usage will be complex, so you can better accommodate your needs with a tailor-made structure, with varying thickness and material choice (including the fibre orientation, which is VERY important, and mostly chosen for you in a tube - see image 3).
Last but not least, your new (but inferior) structure needs to be tested again before the plane can go into service.
There are developments where carbon stringers are used, which is a kinda-yes-but-not-what-you-thought to your question :) (see image 2)

source: https://www.zhaw.ch/de/engineering/institute-zentren/ims/robotik-automation/ecostrippair/

source: https://www.kunststoffe.de/a/news/carbon-ersetzt-aluminium-beim-flugzeugba-4930637

source: https://www.sciencedirect.com/science/article/abs/pii/B9780323993401000071

Please ensure your images are properly attributed

@John K makes many salient points in that to obtain the major advantages of carbon fiber construction you would forego the tube and joint structure in favor of a custom made monocoque structure with limited internal structural members.  The tube and joint method you describe above offer little advantages over a properly designed and welded tube aluminum or tube steel trellis frame construction.
Outside of student engineer musings, it’s often very hard to find applications where the advantages of composite construction outweigh its disadvantages as compared to fabrication using traditional metal alloy construction.

To Add to Johns already great answer, the tubes are also the same size, which would be very hard to design for. Typically you would want tubes of smaller diameter for areas with lower loads. That's not to mention the problem with joining carbon fiber and concentrated loads. Carbon fiber has a low ductility (elongation until break) so a couple bad landings and you're swapping out a bunch of tubes.
Another thing that gets overlooked is bending moment. Take the wing for instance, its very convenient to get material away from the center-line as it contributes more to your moment of inertia (resisting bending). If you have all your mass towards the center in your CHS tubes, its not doing much.

CIvil airliners are already using "Composites" in construction.
Despite it's benefits carbon fiber has a notorious expense to maintain.

Carbon fiber is considerably more expensive than aluminum.
Carbon fiber has lots of rigidity, however it is a brittle substance. YES you can break it.  Since it is a baked fiber product, repairing it is basically a rebuild.
Aircraft need a certain degree of flexibility or creep reflex.
Smaller planes Cessna, 3-10 passenger size props alrady have extensive use of CF. they're more rigid. Lots of planes like 787 Dreamliner, A380 are already moving to new composites.

You can repair carbon fiber, and you probably would have to, if something were to strike your plane. (although it is very labour intensive)  Interesting side note. It is sometimes VERY difficult to gauge the damage to a carbon fiber part from the surface, which is why we test with ultrasound for delamination, as the damage will usually be more extensive on the back-side from the impact, where you wouldn't be able to easily asses it.


text:
Why deplane all passengers and not the troublemaker?
I'm sure you've seen videos like this where they ask everybody to deplane before the police removes an unruly passenger, why?
I remember they use to just remove that passenger alone, why do they need to deplane everybody these days? (this seems to happen only on certain airports, not all of them, but still I don't understand the reasoning)

https://www.ukfsc.co.uk/files/Safety%20Briefings%20_%20Presentations/Cabin%20Safety%20-%20Disruptive%20Passenger%20Guide.pdf - This process doesn't appear to be a formal procedure, at least according to the various flight manuals I've read for UK flights. The goal is to make the unruly passenger leave the plane as a matter of priority

An 'unruly' passenger's behaviour might threaten everyone with arm's reach, at least…
Would it be better to move every potential victim out of the way, or to leave anyone in range?

Deplaning "everybody else" removes the thought that "if I just stay put long enough, eventually they'll give in & depart & I'll go to my destination." If everybody else is off the airplane, the prospect that it will depart for the destination with only that individual on board will seem far less likely -- i.e. it removes the prospect of "I can 'win' by holding out."
But mainly it's so that the troublemaker doesn't have the option of playing to the camera. He's not going to be famous, he's just going to have an arrest record. And trouble getting where he was trying to go, ever again -- probably on airlines' no-fly lists for a fair while.

Neither caring about publicity nor convincing the troublemaker to arrive at a logical conclusion they should surrender are relevant primary motivations for removing the bystanding public from a volatile situation by law enforcement.

If the situation is truly volatile, taking the significant time to deplane one or two hundred passengers, some of them walking right past the troublemaker on their way out, isn't a very good plan.  This is about dealing with the passenger who refuses to  get off the plane when told to, not about how a SWAT team deals with somebody who is violent from the outset.

@RalphJ you can evacuate any aircraft with at most a handful of people having to go past the troublemaker. And while you probably won’t get (and don’t want to get) the <90 seconds of an emergency evacuation, it can still be done in minutes if people don’t take their carry-on. Whether it’s a good idea or useful probably depends a lot on the exact circumstances (I.e. what the trouble is exactly)

A great number of law enforcement agencies, and officers, prefer to gently escalate until they receive compliance. Deplaning alters the environment. For a large number of people, the sudden lack of witnesses is threatening or intimidating. Sometimes altering the environment is enough to obtain compliance, if not, they've only added options to obtain compliance, peacefully or forcibly.

Airplanes are cramped spaces.  Getting everybody else off the plane first gives the police more room to move and fewer bystanders to run into in case the unruly passenger decides to struggle.

Indeed. Basically, one of the reasons they do this is for the safety of the passengers getting deplaned. To avoid collateral damage.

The aircraft needs to be searched after a person is removed and the person's baggage removed from the hold. Otherwise "being a troublemaker" could be used to put a live bomb on the aircraft.

indeed - I imagine that just breaking the "seal" of ending boarding and having a definitive list of who is on the plane might require a re-start in the "security" of today.

For that matter, who's to say what law enforcement brought in and left on board, either by honest mistake or as part of a greater plot.

@Harper-ReinstateMonica I smell a summer blockbuster!

If a troublemaker can bring a live bomb on board in the first place, isn't that rather already a problem? Do you have any source for this being the reason, or at least part of it? (Also, at least here in the U.S., I've never heard of an aircraft being searched after a troublemaker was deplaned, unless the troublemaker was actually making bomb threats or there was some other suggestion that that might be a possibility.)


text:
Why can't we choose our own call sign?
After listening to this funny exchange Kennedy ATC audio on youtube, about how ATC didn't knew Norwegian Airlines callsign was "RedNose", it occurred to me, why not have the option so that every aircraft can use a simple naming structure of just 2 words. Some times I find it painful to listen so many long call signs specially in congested areas.
This is my suggestion:

You would go to the ICAO website to pick the 2 word combination you want tied to your tail number.(the system will tell you if your combination is available/not taken)
ICAO would create a list of approved English words from which one could pick the desired 2 words combination.(the list could be updated annually by ICAO)
Those words would be picked by ICAO based on criterias like simplicity, popular/global recognition, not conflicting(ie words like "airplane", "airspeed", "altitude" shouldn't be allowed), etc.
Airlines could also use this by suffixing one word after their company name(ie. AmericanHappiness, UnitedDreamer, DeltaJupiter, DeltaHumble, etc)
Yes, there are enough English words so that we only have to use 2 words combination for the total aircraft population(For the current system the English alphabet only has 26 letters and 9 numbers in a combination of 6 positions for the tail naming. In contrast, the number of usable words for the proposed system could be up ti 100k with 2 positions)
This would not be a requirement, only a option available to every aircraft owner if they choose to, they could still use their tail number call sign if they want.

I mean seriously guys, don't tell me you want to be called NovemberFourTwoSixAlphaCharlie(N426AC) when instead you could be called:
FreedomLoverSweetMangoBigLionWhiteIndianCuteFace
I think this would be easier to remember, simpler, faster/shorter readbacks,
and more distinguishable.
PS: I'm not proposing to change the tail number naming structure, just the call sign, the same way comercial airlines can have a different call sign than the tail number.

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

Let's clear a few things up. Scheduled flights generally use the airline prefix followed by the flight number as their callsign. So the callsign of United Airlines flight number 345 will be "United three four five".
Many of the airline prefixes are just the shortened form of the airline name - "United" for United Airlines, "Southwest" for Southwest Airlines etc. Some are more obscure (but well-known to pilots and ATC) - "Rednose" for Norwegian, "Speedbird" for British Airways. There are usually historical reasons for the unusual ones. As far as I know these prefixes are chosen by the Airlines when a new airline is created, with the approval of the ICAO. The likely restrictions are on making sure each prefix sounds distinct over the radio and can't be confused with others.
The flight number is also useful information. Controllers at (e.g.) JFK will quickly get to know that "United three four five" is going to Miami in the late afternoon every day. Also note that scheduled flight callsigns don't stick with an aircraft, but a flight. If UA345 is being flown by a different plane today it still has the same callsign.
So for scheduled flights a perfectly good system of callsigns exists that includes very useful information.
For non-scheduled flight, replacing the tail number callsign with an arbitrarily chosen pair of words is removing useful information. The tail number can be traced back to an aircraft, in a database that already exists. You should also remember that except in exceptional circumstances a callsign is truncated to the last two or three numbers after the first exchange, meaning that two arbitrarily chosen words might take longer to say than three numbers. Outside the US the norm is to reduce the callsign to the last two letters - making it exactly as long as two arbitrary words.
ATC (and others) need to be able to trace what is said on the radio back to a known aircraft and do it easily. Your proposal adds an extra step to this that didn't exist before. So that's a disadvantage to be overcome.
P.S. It's a mistake to think that "having a lot of words to choose from" makes things clearer or easier to distinguish. Not all pilots speak English as a first language. It's very possible that an arbitrarily chosen callsign is a word that is unfamiliar to some of the pilots around. Also don't forget that a lot of aviation happens outside of the US. Are Russian or Chinese pilots allowed to choose Russian or Chinese words? What happens when they visit the US? Everybody can learn 26 letters and ten numbers (and some airline names). Learning arbitrary words is much more difficult.

"As far as I know these prefixes are chosen by the Airlines when a new airline is created"  That explains why Jet2 has the callsign 'Channex'.  The airline was originally called Channel Express but rebranded as Jet2 at some point.

Yes though truncated to the last two to three numbers make it exactly as long as two arbitrary words, the difference is that you only have 26 letters plus 9 numbers, while with words you would have a lot, I mean a lot of words to choose from, thus making less likelihood of misinterpretation or hearing "similar call signs"

I think you are completely wrong.

@Gabe Having a lot of words isn't the same as having a lot of distinct, can't-be-confused words. Consider all the times people try to rewrite the phonetic alphabet to make it more 'sensible'

even if you discard all similar words, believe me, you're goanna get a lot higher number of combinations. Apple, Astronaut, Alligator, Amazing  .... I don't think you can confuse any of those, and believe I can give you a much longer list of very simple easy words. Try to compare that to the limited 6 x 26 letters and 9 numbers we currently have.

@DaveGremlin - and of course the wonderful "Cactus" call sign of U.S. Airways. Originating from America West Airlines which started in Phoenix, Arizona. I first heard that call sign decades ago living in Southern California and listening to a portable radio with Air band. I also flew on America West several times, like Southwest back then it was fun to fly on. Interesting item from the Wikipedia article on America West, "In the early years, passengers could purchase their tickets on the aircraft."

"Controllers at (e.g.) JFK will quickly get to know that "United three four five" is going to Miami in the late afternoon every day". ADS-B shows the destination now, I wonder if that information is readily available to controllers, i.e. on their screens. Or do they all have their phones sitting in front of them running the Flightradar24 app?

Small nitpick. The norm outside the US is to reduce callsigns to the first + the last two letters - so 3 letters total, not 2. OYABC would become OBC etc.

That may be the official standard, but just the last two is very common, especially outside major airports. Even more especially given that the first identifies the country and so is shared by virtually all the planes in a given place, so adds nothing useful.

I think, @Gabe, you'd need to find or do research to prove your point that Corolla Apple is more clear and less prone to error, misinterpretation, and confusion than November Tree Two Fife Niner Alpha. Not only for American, British, and other native English speakers, but for a broad cross-section of pilots all across the world. Yes, I specifically picked Corolla because despite being the name of the most common car ever built, the Japanese who named it cannot pronounce the ll and instead pronounce the word Corora. I don't see the benefit in that...

As DJClayworth touched on, your proposal would create too many words to make them standard. Standard phraseology is much easier to learn and also much easier to understand over the radio since you have an idea of what you're listening for. Airline call signs are standard (and common enough in their area) and can be clarified phonetically with three letter ICAO codes. Flight numbers can be spelled out individually. Same with registration numbers. Using random words would get confusing quickly. If they're not understood they would need to be spelled out and that would defeat any benefit of brevity.
I think it would also be a lot more difficult than you assume to exclude the words most often used when talking to ATC. For one, there are almost 70,000 fixes and waypoints just in the FAA jurisdiction that have names. VORs and NDBs used for navigation have their own names. There are also many named instrument procedures for departures, arrivals, and approaches into many airports with many names. Lastly, standard phraseology can only cover so much, so pilots and ATC will sometimes fall back to plain English. All of these cases could create situations of confusion if aircraft start using the same or similar words to identify themselves.
The entire global ATC system is built based on humans identifying a specific aircraft by an ID of only about 8 characters, whether that is a call sign and number or a registration number. It's lot easier for ATC to have UAL485 on their screen than UnitedDreamer or even UALDreamer. It's going to be extremely hard to change all of that unless you have an extremely compelling case with clear benefits for everyone. Even today METARs are still reported in an arcane mix of abbreviations based on both English and French, and NOTAMs have their own abbreviations. Spelling everything out wouldn't even change the meaning. But having a concise format that everyone has learned and used for decades is hard to change.

Ships are a notable exception to the convention of using numbers in that they are primarily identified by their names, but there are several reasons for that tradition:

A ship crew and passengers will live on the ship for voyages of weeks or months,
which gives them a much more personal attachment than most airline crew and passengers boarding a different aircraft every day, or even multiple in a day.
Ships are more unique, especially historically, whereas aircraft are
more mass-produced and a specific one isn't that special.
Even today the commercial ship fleet has an average age of about 20
years, while the airliner fleet age is around 10 years, so the
specific ships will stick around longer.
Though aviation has borrowed some things from the maritime world, the
tradition of naming ships goes back much further than the relatively
new field of aviation.
While there is certainly communication needed to coordinate movements
especially around ports, ships don't have a global system like ATC
that they are always in contact with for navigation and control.

That being said, some airlines do name their aircraft. It's just informal and the airplane is still primarily known by its registration and flight number.


text:
What are the reasons to use a downward vertical stabilizer?
Came across this AGM-129_ACM missile and noticed its downward vertical stabilizer and then I also recalled seeing other aircraft (just can't recall which one) with downward vertical stabilizer as well.
What are the reasons to use this unconventional design on an aircraft and/or what would be the reasons this manufacturer chose this design?

source

It's written in the Wikipedia entry: "The jet engine exhaust was shielded by the tail and cooled by a diffuser to reduce the infrared signature of the missile"

At least in the F-16, is has both an upward-facing vertical stabilizer, and a smaller downward facing one. When at high angles of attack, the flow over the upward facing one can become separated, so a stabilizer on the bottom was needed.

If it's air launched a vertical fin could interfere with the rack it mounts to...

Tail is easier to reach, for short maintenance staff ?

@MichaelHall if it's air launched it could half roll short after launch and proceed its task with that yaw stabilizer pointing wherever it points

@user721108, Sure, and then somebody asks "why does it do a half roll after launch instead of just flying with the stabilizer pointing down?"

The question must rather be "why are vertical stablizers usually facing upward?", with a rather obvious answer ;-).

The answer provided by user71659 is very good and answers the radar-detection aspects.
One of your big questions, however, is why this unconventional design?  The answer is that most aircraft are designed to safely land on a flat surface.  Downward vertical stabilizers generally interfere with landing gear.  Missiles, on the other hand, are designed to never return to base (but occasionally do so when they malfunction or are improperly programmed).  Because they don't have to be concerned about landing, their designs do not need to adhere to typical aircraft conventions that require landing accommodations.
Given that the AGM-129 ACM was used to deploy nuclear warheads, landing was not a concern.
In some cases, as Michael Hall alluded to in the comments, an upward vertical stabilizer could interfere with mounting hardware or launch aircraft flight surfaces.  The B-52H was always used as the launch aircraft for the AGM-129 ACM, but the flight surfaces of the AGM-129 ACM would be retracted pre-flight to prevent interference with the mounts and flight surfaces of the Stratofortress.
A typical 3-cluster mount configuration looked like this (thanks to Sophit for providing the image link):

image source: https://commons.m.wikimedia.org/wiki/File:AGM-129A_-_2006_0306_b52_2lg.jpg

Sorry but this answer isn't that correct: When they are stored or attached to the launcher airplane (B-52 for example), wing and empennage are normally folded no matter where they are placed, as it's just the case for the AGM-129 in the question. The correct answer is therefore the one of @user71659

@sophit Thanks. For some reason, I thought only the lateral stabilizers were retracted until deployment, not the vertical one.  Thanks for pointing out my error; I'll significantly edit that paragraph.  It is relevant for other missiles, but not the AGM-129. Thanks again.

Perfect, glad I could help

There is one pertinent line from this answer.  The missile is not intended to land.  Most vertical stabilizers  are upward to facilitate landing on a runway.  Even though the wings/stabilizer are retracted initially, it is highly doubtful they are designed to retract once launched.

@Gabe: there are many pictures and videos online where you can see how it works

@sophit thanks for suggesting a video search. Interestingly found this and looks like the vertical stabilizer is never folded, the wings definitely are though. I think @ AmazonDiesInDarkenss initial point is good and would make sense even though if the missile is launched from the B-52H, because I guess it would allow to be launched from other aircraft as well while reducing the complexity(and cost) of the missile by having one less part that needs to be folded.

The reason is to reduce radar detection from above.
Cruise missiles are designed to fly low, to avoid detection. Low altitudes make it harder to detect because the radar horizon is smaller, and things on the ground, like terrain and trees, get in the way of the signal.
The way you counter a low-flying vehicle is to detect it from above, via a radar on an aircraft.
Therefore, stealth features on the AGM-129 were designed to reduce the radar cross section from above, which resulted in features like the vertical stabilizer and air intake being placed on the bottom.
A similar layout is used on the Storm Shadow.

The bottom mounting also had the aerodynamic benefit of providing a favourable rolling moment to help the elevons when rolling into a bank.

Just adding to the above: "Not just the wings were backward, all the low observable features were oriented towards defeating detection from above, not below. This resulted in an upside-down-looking airframe of sorts. " https://www.twz.com/31286/the-saga-of-the-agm-129-cruise-missile-that-was-basically-a-stealth-jet-designed-upside-down

Here's a Dornier DO-335 Pfeil from WW2.

The vertical stabiliser hanging down also did dual duty to protect the rear pusher prop from damage.

Since your question is asking about vertical stabilisers generally and was not specific to the example missile, here's a historical reason why a rudder might hang down.

Good example, up voted.

A big reason missed, is that this would prevent deep stall of your vertical stabilizer. Where the vertical stabilizer loses its functionality because of the turbulent air from a stalled main wing.
Typically we consider stall in a high angle of attack situation (the reverse, stall happening at a high negative angel of attack is mostly indication of other major problems).
With high angle of attack the tail is "lower" than the main wing, so a vertical stabilizer high up on the tail would be more likely to be in the disturbed air flow of the main wing.